1. Field of the Invention
The present invention relates to a process for the purification, by supercritical extraction, of crude vanillin, especially crude vanillin obtained from paper mill waste liquors.
2. Description of the Prior Art
Vanillin is the common name for 4-hydroxy-3-methoxybenzaldehyde.
Vanillin is the principal flavoring in vanilla, and for hundreds of years had been obtained by processing the seed pods of vanilla planifolio and vanilla fragrans. The seed pods were crushed and fermented and the vanillin extracted by alcohol.
Vanillin has also been prepared by processing parts of conferous trees. Coniferon was extracted, then oxidized, and finally hydrolyzed to vanillin. See U.S. Pat. No. 151,119 (May 19, 1974), the teachings of which are incorporated herein by reference.
Vanillin has also been synthesized from eugenol, guaiacol, and safrole.
Most vanillin today is obtained from paper mill waste liquors. It was recognized as early as 1875 that the waste sulfite liquor contained some vanillin. The problem was concentration and purification of the crude vanillin in the paper mill waste liquors.
Commercially, vanillin is produced by the controlled oxidation of lignosulfonates, usually obtained from waste liquors.
A general scheme for the production of vanillin from waste sulfite cooking liquors involves adding the waste sulfite liquor, air, and NaOH to a reactor. This step is taught in U.S. pat. No. 2,692,291, the teachings of which are incorporated by reference.
The crude product, containing very dilute vanillin, may then be extracted with a solvent, e.g., 1-butanol, for removal of lignin salts. The somewhat purified vanillin solution is then subjected to bisulfite extraction by contacting the mixture of crude vanillin and 1-butanol with NaHSO.sub.3 in a bisulfite extractor. The aqueous phase is then treated with H.sub.2 SO.sub.4 and air in a blow tower to produce SO.sub.2 and crude vanillin. The crude vanillin is then subjected to vacuum distillation, with the crude vanillin being recovered in the purified form overhead. Final purification is obtained by multiple crystallization. Such extensive and expensive purification steps are necessary because along with the vanillin are produced such compounds as acetovanillone, p-hydroxybenzaldehyde, 5-formyl vanillin, and syringic aldehyde. This general scheme for the production of vanillin is disclosed in Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd. Edition, Volume 21, pp 180-196.
U.S. Pat. No. 3,049,566, the teachings of which are incorporated by reference, teaches purifying crude vanillin obtained via lignin oxidation by crystallization using a methanol-water solvent.
U.S. Pat. No. 3,600,442, the teachings of which are incorporated by reference, teaches treating the crystallization liquors with an alkali metal hydroxide and a zinc or magnesium salt to precipitate vanillin and acetovanillone. The vanillin is then separated from the acetovanillone by the bisulfite method.
A number of improvements have been suggested for extracting vanillin from the lignin liquor. U.S. Pat. No. 3,686,322, the teachings of which are incorporated by reference, teaches using a hot hydrocarbon solvent to extract the vanillin, then cooling the solvent to crystallize the vanillin.
The current processes for purifying crude vanillin are fairly successful, and perhaps inevitable, as far as concentrating the crude vanillin from the 1 to 15 weight percent range, as found in the waste liquor, and concentrating the vanillin. There is nonetheless a significant amount of energy expended in the final purification of vanillin, i.e., taking it from the 50 to 95 percent purity range to the 98-99+ percent purity range. The final purification step is very difficult because everything that is easy to remove from the crude vanillin, e.g., water, has already been removed in prior processing steps. The materials that remain with the vanillin have very similar chemical and physical properties such that conventional fractionation techniques are utterly inadequate, and only multistage crystallization has found favor in obtaining product of the desired high purity.
Some attempts have been made to bypass the problems of prior art methods. One early attempt involved the use of liquid CO.sub.2 to extract flavors from natural spices. U.S. Pat. No. 3,477,856, the teachings of which are incorporated by reference, taught that liquid CO.sub.2 could extract flavor essences from all kinds of materials which contain volatile flavoring components. Specific examples included spices such as pepper, cinnamon, bay, and also tea, coffee, cocoa, meats, and other materials of plant or animal origin. Liquid CO.sub.2 was used as a solvent and the flavoring materials were recovered by vaporizing the liquid CO.sub.2.
The use of CO.sub.2 was taken one step further in U.S. Pat. No. 4,198,432, the teachings of which are incorporated by reference. The patentee found that it was possible to extract flavor and aroma constituents from natural spices selected from the group of black pepper, cloves, cinnamon, and vanilla by dry supercritical extraction followed by wet supercritical extraction. The dry supercritical extraction dissolved the aroma constituents from the spice, while the wet supercritical extraction dissolved the flavor constituents of the natural spices.
Unfortunately, the problems encountered in extracting flavor and aroma components from natural spices are quite different than those encountered in extracting vanillin from aqueous streams. For one thing, many of the flavor and aroma constituents in natural spices are not water soluble, whereas the vanillin in spent sulfite liquors is in water solution. None of the prior art methods of purifying vanillin obtained from spent cooking liquors were completely satisfactory as they required a lot of energy and effort to recover vanillin, at great purity, from a relatively dilute solution. The use of CO.sub.2, either as a liquid or supercritical fluid, for processing of natural spices did not provide much guidance because the starting materials were different, and these methods taught extracting small amounts of flavor and aroma components from large amounts of vegetable matter. All of these prior art methods missed the target because they focused on recovering the vanillin rather than removing the impurities.
I have discovered an economical and effective way of purifying crude vanillin derived from lignin and similar materials. My method is economical because it extracts the impurities, leaving as a valuable residue purified vanillin.